The invention relates to a power supply circuit and to a switched-mode power supply with a power supply input circuit.
Power supply circuits serve to connect a load to an electricity power supply, in particular to the respective electricity power grid.
Known power supply circuits for supplying a DC voltage to a load comprise in this respect a connection to an AC voltage supply, e.g. the electricity power grid, and a rectifier supplied with the AC voltage. In addition, the known power supply circuits comprise further components, including at least a first smoothing capacitor. This is a capacitor with a high capacitance value, often an electrolytic capacitor, which is conventionally connected to the load terminals in parallel with the load and is charged by the rectified voltage or discharged via the load. This capacitor thus effects smoothing of the output voltage. In addition, the power supply circuit optionally comprises further components, for example a filter impedance and a resistor for limiting the current peak.
Simple power supply circuits may serve to deliver an unregulated voltage for an consumer of electricity. However, power supply circuits may also occur as components of other circuits, in particular as the power supply input circuit of a switched-mode power supply. Switched-mode power supplies operate with at least one converter supplied with an intermediate circuit voltage, which converter conventionally supplies a transformer. By suitable actuation of the converter, regulated output voltages may be produced. In such a switched-mode power supply, a power supply circuit serves, as described above, to deliver the intermediate circuit voltage. In this case, the subsequent modules of the switched-mode power supply are connected to the load terminals as a xe2x80x9cloadxe2x80x9d. In order to be able more easily to summarize these two possible uses for a power supply circuit in the following text, the designations xe2x80x9cloadxe2x80x9d and xe2x80x9cload terminalsxe2x80x9d are used below even if reference is being made therewith to downstream modules of a switched-mode power supply.
Dimensioning of the components of a power supply circuit, especially the smoothing capacitor, proceeds with reference to predetermined specifications for the circuit. They include input-side specifications for the AC input voltage (minimum, nominal and maximum values for voltage and frequency together with details about specified voltage interferences) and output-side specifications for the load (minimum, nominal and maximum values for current, voltage and power consumption). So as to meet these specifications in all respects, the power supply circuit must be so designed that it still meets the predetermined specifications even in the least favorable operating range for the parameters under consideration (worst case).
With respect to the smoothing capacitor, for example, this means that on the one hand the dielectric strength of the capacitor used has to be so selected that it is sufficient even when the supply voltage assumes the maximum permissible value and at the same time no output power is drawn. On the other hand, the capacitance value of the smoothing capacitor has to be sufficiently high for a sufficiently smoothed voltage to be present at the load terminals. If so planned by the requirements of the circuit, it must additionally even be ensured that, in the event of specified power failures (e.g. 100% power failure for a duration of 50 ms), the minimum output voltage necessary remains ensured even in the event of maximum power being drawn.
If the AC input voltage covers a wider voltage range, a capacitor which exhibits both high dielectric strength and a high capacitance value is often required to fulfill all these requirements. This leads to the use of structural elements of enormous size, which are also correspondingly expensive. If a power supply circuit is to be suitable throughout the world for connection to the electricity power grid (world-wide mains), rms AC input voltages ranging from a minimum of 85 V (specified minimum in USA) to 265 V (specified maximum in Europe) have to be expected.
U.S. Pat. No. 4,001,668 describes an electric razor with a power supply circuit. For operation at different AC input voltages, a switching unit is provided which supplies a load (motor) from a unidirectionally rectified power input line voltage. A filter capacitor is connected in parallel with the load. The load voltage is regulated to a value below a voltage threshold, wherein the voltage threshold lies below the supply transient overvoltage. This circuit allows operation of an electrical consumer at different AC input voltages.
U.S. Pat. No. 4,811,190 describes a circuit for extending operation of an electrical load after interruption of the power supply. The circuit is supplied by a direct current source and includes storage capacitors which are charged by the DC voltage source and kept at a predetermined voltage by means of a regulating circuit. If the power supply is interrupted and the voltage drops below a threshold, a switching device with a diode and an SCR (Silicon Controlled Rectifier) ensures that the storage capacitors are connected in series and this series circuit, in parallel with the optional filter capacitors, is arranged in parallel with the load and supplies it with power, while the filter capacitors are discharged.
It is an object of the invention to provide a power supply circuit and a switched-mode power supply with a power supply input circuit, which is suitable for operation over a wide range of voltages.
According to the invention, at least a second capacitor is provided. This second capacitor is connected by a switching element to the load terminals so that it is connected only when the voltage at that point drops below a voltage threshold.
This may be a fixed voltage threshold, but it is preferable for the switching element to operate as soon as the voltage at the second capacitor is higher than the voltage at the load terminals. For this purpose, a diode may be used as switching element.
The second capacitor is preferably a component with a capacitance value that is at least as high as but preferably higher than the capacitance value of the first capacitor. A capacitance value is proposed for the second capacitor which corresponds to at least twice the capacitance value of the first capacitor, preferably to more than five times, preferably to approximately ten times the capacitance value. It is sufficient in this respect for the dielectric strength of the second capacitor to be at or just above the voltage threshold. The second capacitor is preferably operated at most with a voltage corresponding to a charging threshold which is clearly below the maximum admissible value, according to the specifications, of the voltage at the first capacitor. A component may therefore be used which exhibits a lower dielectric strength (in comparison with the first capacitor), which reduces component costs and structural size.
To simplify the explanation, mention will here always be made of a first and a second capacitor. However, it is known to the person skilled in the art that, instead of using individual capacitors, it is also possible to produce such capacitor elements using in each case circuits with a plurality of capacitors.
According to a further embodiment of the invention, an apparatus is provided for charging the second capacitor. The apparatus for charging the second capacitor is preferably supplied with the rectified voltage. However, it is also possible to use a separate energy source for supplying the charging apparatus. Advantageously, the charging apparatus may comprise an apparatus for limiting the voltage at the second capacitor to a charging threshold, for example a zener diode.
A charging apparatus is preferred which comprises a switching unit by means of which the second capacitor may be connected to the rectified voltage. Charging of the second capacitor is performed in that the switching unit connects the capacitor to the rectified voltage if the voltage at the capacitor drops below a maximum charging threshold. The charging threshold serves to ensure that the second capacitor is not operated above this voltage value, so that a component with a correspondingly low dielectric strength may be used.
If the switching unit connects the second capacitor to the rectified voltage, the second capacitor is charged. This applies at any rate as long as the rectified voltage is higher than the voltage at the second capacitor, which may optionally be ensured by suitable means. Charging of the second capacitor is performed with a charging time constant, which depends to a considerable extent on the incoming resistance, and here in particular on the volume resistance of the switch. It is therefore preferable to use a switch with low resistance, for example an FET. If the voltage at the second capacitor reaches the charging threshold, the switching unit disconnects the second capacitor.
Selection of the voltage threshold is of decisive significance for the operating behavior of the power supply circuit. In the event of the voltage threshold being set very high, i.e. if the maximum possible rectified voltagexe2x80x94according to the specificationsxe2x80x94is below the voltage threshold, the second capacitor is always connected by the switching unit, so that the arrangement would correspond to a parallel connection of the first and second capacitors. In this case, however, the switching unit is pointless, so that it is recommended that a voltage threshold be selected that is below the maximum possible rectified voltage according to the specifications. Since the degree of savings that can be achieved with regard to component costs and structural size of the second capacitor is determined by how far the voltage threshold is set below the maximum possible rectified voltage, a threshold voltage is preferred that is clearly, preferably at least 25%, particularly preferably even more than 50%, below the maximum possible voltage.
According to a further embodiment, it is recommended that the voltage threshold be selected so that it is at or above the peak value of the minimum input voltage still admissible within the specifications. For example, for worldwide mains operation, the minimum voltage amounts to 85 V (rms value), so that the voltage threshold here should be 120 V or higher.
On the other hand, however, selecting an extremely low voltage threshold leads to the second capacitor becoming active in an extreme case only if there are relatively long line failures, since only in the case of a relatively long supply voltage failure does the rectified voltage, smoothed by the second capacitor, at the load terminals drop so far that it drops below the voltage threshold. In this case, the second capacitor then merely serves as a buffer for such line failures.
Such behavior may be desirable. However, it is preferable to select a voltage threshold between the two above-mentioned extremes, so that, although on the one hand distinct savings are achieved with regard to component costs and size, the threshold voltage is set high enough for it to become active according to the specification in steady-state operation.
In a conventional instance of operation, the AC input voltage is approximately constant with regard to amplitude for an observed period. The rms value is within the specified range, for example 85-140 V for the electricity power grid in the USA, 189-265 V for the European electricity power grid or 85-265 V for worldwide operation. When a consumer of electricity with a given power consumption is connected, a time profile for the output voltage is then established in steady-state operation at the output terminals, which time profile has a cycle of twice the mains frequency in the case of four-way rectification. In the case of a sinusoidal input voltage, a voltage shape is produced at the smoothing capacitor which follows the shape of the rectified voltage in the form of a xe2x80x9cfolded upxe2x80x9d sine curve, is charged by the first capacitor in the maximum value range and discharges in the zero value range. When only a single smoothing capacitor is used, an output voltage would thus be obtained, the ripple content of which would depend on the capacitance value of the first capacitor.
According to a further embodiment of the invention, it is proposed to use the second capacitor according to the invention with a voltage threshold that is selected so that the voltage at the load terminals is lower at least once per cycle than the charging threshold, so that the second capacitor is active. This covers instances at which the voltage threshold is set so high that the second capacitor is constantly active at typical input voltages. However, it is also possible to select the threshold so that the voltage at the load terminals cyclically drops below the voltage threshold, so that the second capacitor becomes active cyclically. If the voltage threshold is suitably selected, this leads to a less rippled voltage at the output terminals, since, as soon as the voltage drops below the given voltage threshold, the second capacitor is connected, so that a substantially flatter discharge curve is obtained. Up to the next charging process, through the following half-wave the voltage merely drops to a minimum value, which is markedly higher than the minimum value when only the first capacitor is used. Thus, use is made of the second capacitor with regard to voltage smoothing, without a dielectric strength being required which covers the entire input voltage range.
A further embodiment of the invention relates to the choice of voltage threshold especially for operation at two voltage levels, for example world-wide mains operation, with two typical input voltages, for Europe (189 V/230 V/265 V), on the one hand, and the USA (85 V/117V/140 V) on the other. To operate the appliance on the basis of these input voltage stipulations, it is proposed to set the voltage threshold in the range between the peak value of the minimum voltage of the lower voltage level and the peak value of the maximum input voltage of the lower of the two operating voltage levels. For worldwide mains this means a voltage threshold between 120 V (this is the peak value, that is obtained in the case of the minimum rms input voltage of 85 V) and 200 V (corresponds effectively to 140 V, maximum rms value of the US input mains voltage).